If you want to treat it like a number, then assigning text like "Not Yet
Approved" is going to foul it up. Better to leave it as Null, and then put
that into the text box where it will be displayed. For that result, try:
TimeToIssue: DateDiff("d",[REceivedTransdate],
Nz([ApprovedStandardTransDate], [ApprovedSubStandardTransDate])

If you don't care that is is treated as text (e.g. 2 sorts after 10), try:
TimeToIssue: Nz(DateDiff("d",[REceivedTransdate],
Nz([ApprovedStandardTransDate], [ApprovedSubStandardTransDate]), "Not Yet
Approved")

If you want to treat it like a number, then assigning text like "Not Yet
Approved" is going to foul it up. Better to leave it as Null, and then put
that into the text box where it will be displayed. For that result, try:
TimeToIssue: DateDiff("d",[REceivedTransdate],
Nz([ApprovedStandardTransDate], [ApprovedSubStandardTransDate])

If you don't care that is is treated as text (e.g. 2 sorts after 10), try:
TimeToIssue: Nz(DateDiff("d",[REceivedTransdate],
Nz([ApprovedStandardTransDate], [ApprovedSubStandardTransDate]), "Not Yet
Approved")